1. Field of the Invention
This invention is directed toward the determination of radial dimensions or "caliper" of a borehole penetrating earth formation, and more particularly directed toward determining caliper by irradiating formation with neutrons and measuring neutron flux within the borehole. The invention can be embodied to measure caliper while the borehole is being drilled, or alternately embodied to measure caliper as a wireline logging system used after borehole drilling has been completed.
2. Description of the Art
Accurate borehole caliper data is important for both the drilling of a well borehole, in the measurement of earth formation parameters penetrated by the borehole, and in completing the well after drilling.
In drilling a typical borehole for hydrocarbon production, the drill string is formed from sections or "joints" of drill pipe which are added to the drill string by threaded collars, and which is terminated by a drill bit. The drill string is rotated my means well known in the art, and the borehole is advanced by the cutting action of the drill bit. Drill bits must be periodically replaced as they become dulled by the drilling action. Bit replacement requires that the drill string be pulled or "tripped" from the borehole by sequentially removing joints of drill pipe. Borehole caliper data from successive trips in the borehole can be used to monitor wellbore conditions such as early indications of borehole washout and impending wellbore instability. Caliper information can allow a driller to take remedial actions during the drilling operation to prevent damage or catastrophic loss of the borehole, of drilling equipment, and even the loss of life of drilling personnel.
Formation parameter measurements as a function of depth, commonly referred to as formation "logging", can be made subsequent to the drilling of the borehole by instruments conveyed by wireline, or can be made while drilling the borehole by instrumentation conveyed by a drill string. These techniques are commonly referred to as "wireline logging" and "logging-while-drilling" or "LWD", respectively. Wireline and LWD measurements use borehole caliper data to correct measured parameters for various effects related to the radial dimensions of the well borehole. As examples, responses of most prior art neutron porosity, scatter gamma ray density, and resistivity type logging systems are functions of borehole size and must be corrected for borehole size effects to obtain optimum measurements of the desired formation parameters.
Once the drilling of a borehole is drilled to the desired depth, it is "completed" typically with a string of steel casing around which cement is pumped thereby filling the casing-borehole annulus. Caliper information is very useful in determining completion requirements, such as the amount of cement required to properly cement casing.
Many wireline and LWD systems are designed to minimize the effects of borehole size. The basic methodology utilizes two or more axially spaced sensors in the downhole "tool" portion of the system. Each sensor responds in a different degree to borehole size, and the responses are combined to minimize borehole effects. As an example, a dual detector neutron porosity wireline system was introduced in the 1960's in an attempt to minimize the effects of the borehole upon the measurement of formation porosity. Such a system is described in U.S. Pat. No. 3,483,376 to S. Locke issued Dec. 3, 1963. Two thermal neutron detectors are spaced axially at different distances from the source of fast neutrons. The ratio of the responses of the two detectors varies with formation porosity, yet is somewhat less sensitive to borehole parameters than the count rate from either of the two individual detectors. The ratio is, therefore, the measured parameter used to compute porosity. Corrections are made to the porosity value computed from the ratio in order to improve accuracy. Although much smaller than for single detector systems, borehole diameter corrections for dual detector systems are significant and can be quantified if an effective borehole caliper is available. More sophisticated algorithms have been used to combine sensor responses. Again using a dual detector neutron porosity system as an example, U.S. Pat. No. 4,423,323 to Darwin V. Ellis and Charles Flaum, issued Dec. 27, 1983, applies what is commonly known as the "spine and rib" interpretation to the count rate of each neutron detector in order to obtain a borehole size invariant porosity measurement without using an independent borehole caliper signal. The algorithm is relatively complex, and the range of borehole diameter variation over which reliable compensation can be obtained is relatively limited.
Various types of wireline borehole calipering devices were, and today still are, run in conjunction with borehole size sensitive wireline logs to provide a measure of borehole diameter from which borehole size corrections are computed. One type of caliper is obtained from an articulating arm of a pad type tool such as a pad mounted scattered gamma ray density tool, which was introduced commercially in the 1960's and is well known in the art. This type of caliper measures only one radial dimension, which is typically the major radial axis in a non-round borehole. Other prior art wireline calipers utilize measurements from multiple arm devices. These devices can be "stand-alone" caliper tools. Alternately, borehole caliper information can be obtained from arm positions of other types of logging tools such as multiple-arm formation dip tools. Although yielding a more representative measure of borehole size than a single arm device, multiple-arm devices are notoriously complex mechanically, difficult to operate effectively in harsh borehole conditions, difficult to maintain calibrated, and expensive to fabricate.
Prior art LWD systems, like their wireline counterparts, are sensitive to borehole size. Accurate caliper information is required to properly correct parametric measurements from these systems. It is readily apparent that arm type wireline calipers are not applicable to LWD since the drill string is typically rotating, and the arms engaging the penetrated formation would be quickly severed by this rotational movement. Other basic approaches must, therefore, be applied to LWD calipering.
Various methods have been used to obtain borehole size in LWD systems. Estimates can be obtained from the drill bit diameter, the drilling fluid pumping pressure, and the mechanical properties of the formation being penetrated. This method, at best, provides only a rough estimate of a borehole caliper in the vicinity of the drill bit since formation and drilling mechanical conditions can change rapidly. Other methods have been employed in an attempt to reliably caliper the borehole without using a specifically dedicated LWD caliper system. Generally speaking, these methods combine data from a plurality of LWD devices which exhibit different sensitivities to borehole geometric parameters. Such additional LWD devices might include well known scattered gamma ray density devices and resistivity devices which respond to varying radial depths of the borehole and formation environs. Borehole information is extracted by combining responses of these devices, and borehole corrections are derived from these responses. Again, generally speaking, this method of calipering a borehole and correcting measurements for borehole effects is not reliable. In addition, a relatively complex suite of LWD devices must be employed in order to practice this method.
U.S. Pat. No. 5,175,429 to Hugh E. Hall. Jr. et al, issued Dec. 29, 1992, addresses borehole calipering as a tool stand-off compensation method for nuclear LWD measurements. No independent borehole caliper or any other subsystem is required to obtain the desired tool stand-off or borehole size compensation. Count rates from a plurality of nuclear detectors are sorted and stored in "bins" as a function of apparent instrument stand-off. Detector responses are examined as a function of energy level thereby requiring spectral recording capabilities in the borehole instrument. These required features greatly increase the complexity of the borehole instrument, increase the demands on the logging-while-drilling telemetry system, and necessitate a relatively complex interpretation algorithm.
Most prior art LWD systems dedicated specifically to borehole calipering typically employ acoustic methods. More specifically, acoustic methods have been employed in order to obtain an improved measure of the position of the borehole wall in the vicinity of neutron porosity and other LWD systems which might require a borehole size correction. The dedicated borehole acoustic caliper typically emits high frequency acoustic impulses radially from one or more transducers positioned on the periphery of the LWD instrument. These acoustic signals traverse intervening drilling fluid, are reflected at the borehole wall, and again traverse intervening drilling fluid as part of the energy returns to the LWD instrument. The time between the emission of the acoustic pulse and the detection of the reflected pulse is measured. If the acoustic properties of the drilling fluid are known, the distance to the borehole wall can be computed from the measured travel time. Compared to the previously discussed method, this is a more accurate and precise means for "calipering" the borehole. There are, however, disadvantages. The acoustic caliper methodology requires an additional LWD system which is relatively complex and which must operate in the harsh drilling environment. This decreases reliability, increases operational cost, and increases the manufacturing cost of the LWD assembly. Furthermore, any type of reliable acoustic measurement is difficult to obtain in the acoustically "noisy" drilling environment. Still further, once a radial profile of the borehole is obtained, this measurement must be processed mathematically in order to obtain a borehole correction for a specific LWD system matching radial profile to an azimuthal response factor of the system.
U.S. Pat. No. 5,767,510 to Michael L. Evans, issued Jun. 16, 1998, discloses a borehole invariant porosity system, and is hereby entered into this disclosure by reference. The system, which can be embodied as a LWD or a wireline system, is directed toward providing a borehole size invariant neutron porosity measurement using only the responses of "near" and "far" spaced detectors from a source of fast neutrons. No independent borehole caliper measurement is required. As discussed previously, the perturbing effects of borehole size, borehole shape, and the radial position of the instrument within the borehole is overcome, at least to the first order, by computing porosity from a simple ratio of the detector responses. This ratio method does not, however, provide complete borehole size compensation. Additional compensation for borehole effects is obtained by modifying the simple ratio of the near detector to far detector count rates. A function of the far detector count rate has been found that results in a near detector response and a modified far detector response which exhibits nearly identical apparent radial sensitivities over the normal operating range of the tool. The result is a "modified" ratio of near detector count rate to modified far detector count rate that varies with formation, but that is essentially insensitive to radial perturbations such as variations in borehole diameter. Although porosity measurements produced by the Evans system require no caliper for correcting porosity measurements for borehole size, the system disclosed no means for generating a caliper log from the response of the tool.
In view of the previous discussion of background, an object of the present invention is to provide a borehole caliper system which requires no articulating mechanical arms, and which can be embodied as a LWD and a wireline system. A further object of the present invention is to provide a caliper measurement which can be obtained from the responses of one or more sensors deployed in LWD or wireline logging systems and used to make other measurements of properties of formations penetrated by a borehole. Yet another object of the invention is to utilize the response of neutron detectors in a dual detector neutron porosity system to simultaneously generate a formation porosity measurement, corrected for borehole size, and subsequently use the corrected formation porosity in obtaining a borehole caliper log.
Another advantage of the present invention is to provide a borehole caliper log from the response of one detector of a neutron porosity system combined with formation porosity independently measured with another type of LWD or wireline system which measures the neutron porosity of the formation. Still another benefit of the present invention is to provide a borehole caliper log from a detector responsive to hydrogen index combined with an independent measure of formation porosity. There are other objects and applications of the present invention that will become apparent in the following disclosure.